There are known apparatuses for controlling first and second inverters according to a common system voltage input to the first and second inverters to thereby control electrical driving of each of first and second alternating-current (AC) motors as an example of rotary electric machines. One of these control apparatuses is disclosed as a rotary electric-machine control system in Japanese Patent Publication No. 5067325, which will be referred to as a typical patent document.
The rotary electric-machine control system, which will be referred to simply as a control system, disclosed in the typical patent document determines whether the following first and second conditions are satisfied:
(1) The first condition is that the first AC motor electrically connected to the first inverter is operating in a known rectangular control mode, i.e. a known single-pulse control mode, based on torque-feedback control, and the second AC motor electrically connected to the second inverter is operating in a known pulse-width, modulation (PWM) control mode based on current-feedback control
(2) The second condition is that the electrical-angle period of the second AC motor is six times greater than the electrical-angle period of the first AC motor.
In other words, the second condition represents that the electrical-angle frequency of the second AC motor is one-sixth the electrical-angle frequency of the first AC motor.
That is, the ratio of the electrical-angle period of the second AC motor operating in the PWM control mode to the electrical-angle period of the first AC motor operating in the rectangular control mode becomes 6:1.
Skilled persons in the art know that the PWM control of the second AC motor results in the sixth-order torque ripple being generated based on the sixth-order harmonic components of the electrical-angle frequency of the second AC motor.
The ratio of the period of the sixth-order harmonic components of the electrical-angle frequency of the second AC motor to the period of electrical-angle frequency of the second AC motor becomes 1:6. This results in the period of the sixth-order harmonic components of the electrical-angle frequency of the second AC motor substantially synchronizing with the electrical-angle period of the first AC motor. The sixth-order torque ripple based on the sixth-order harmonic components therefore synchronizes with the electrical-angle period of the first AC motor.
That is, establishment of both the first and second conditions causes the sixth-order torque ripple based on the sixth-order harmonic components to oscillate the common system voltage input to the first AC motor in synchronization with the sixth-order torque ripple. The oscillating common system voltage input to the first AC motor causes a rectangular AC voltage, i.e. a single pulse voltage, for driving the first AC motor to also oscillate in synchronization with the common system voltage. This causes the waveform of the rectangular AC voltage to offset from its original waveform, reducing the controllability of the first AC motor operating in the rectangular control mode.
To address the controllability reduction of the first AC motor, the control system is configured to reduce one of a feedback control gain and the frequency of a carrier signal, which are required to control the second AC motor operating in the PWM control mode, upon establishment of both the first and second conditions. This configuration causes the sinusoidal waveform of the sixth-order harmonic components of the electrical-angle frequency of the second AC motor to be disturbed, thus reducing adverse effects, which are based on the sixth-order harmonic components of the electrical-angle frequency of the second AC motor, on the controllability of the first AC motor.